The availability of plant primary metabolites such as proteins, sugars, vitamins, and minerals can affect the ability of phytophagous insects to utilize a hostplant. One way dietary deficiencies of these compounds may reduce insect performance is by affecting activities of detoxification enzymes, thereby altering susceptibility to host plant allelochemicals. In this study I examined the influence of antioxidant vitamins, minerals, and compounds that affect mineral bioavailability on performance and detoxification in phytophagous insects exposed to prooxidant secondary chemicals. In Trichoplusia ni, a generalist herbivore, vitamin A ameliorates phototoxicity caused by ingesting citral, an insecticidal monoterpene. As an antioxidant, vitamin A may protect against UV-mediated citral damage by scavenging free radicals or quenching singlet oxygen. Vitamin C, a water-soluble antioxidant, had a negative impact on cytochrome P450-mediated metabolism of furanocoumarins in this insect when incorporated into an artificial diet. The mechanism of this inhibition remains unknown. In contrast, vitamin C had no effects on growth parameters or P450-based metabolism in Depressaria pastinacella, an oligophagous species restricted to members of the furanocoumarin-rich Apiaceae. Cytochrome P450 monooxygenases may be especially vulnerable to deficiencies in dietary iron because iron is a required component for normal P450 function. For D. pastinacella, an absence of dietary iron significantly reduced growth rate by approximately 60%, but had no effect on P450 function. That detoxificative metabolism is maintained, apparently at the expense of growth, indicates the importance of allelochemical detoxification in this species, even in the context of iron deficiency. Phytic acid, a mineral chelator present in developing seeds of the hostplants of this species, significantly reduced growth and inhibited P450 function, likely through a mechanism other than dietary iron chelation. In a fruit-feeding insect that frequently encounters large quantities of phytic acid (Heliothis virescens) and a generalist folivore that occasionally encounters phytic acid, ( Trichoplusia ni), phytic acid reduced growth and consumption rates, indicating an antifeedant effect. No differences in P450 metabolism of furanocoumarins were observed.